Romans 4 I think has Paul saying that the law which is the mosaic covenant (deut 5:1-3) was 430 yrs after the Abrahamic covenant and could not anull the Promise. If the Abrahamic Covenant was the "old covenant" which Hebrews talks of as being ready to pass away, then it is anulled by completion. Not so with the everlasting covenant of Mercy. . Just because Paul calls the Abrahamic covenant a 'Covenant' and 'the Promise' and calls the Mosaic covenant, the 'law" does not mean that the Mosaic 'law' was not also a (conditional) Covenant and thus the 'old' covenant that was decaying and falling away. Yes, the parts that were promiesed and prophesied to Abraham concerning the nation and his physical lineage were also fulfilled and ended, but the Promise of justification by faith, of blessing/cursing and of all nations being blessed through Abraham (and his pattern of justification by faith) continue.
See also 2 Cor 3
Thus Paul also along with Jeremiah and Daniel indicates that the 'old' covenant to which Hebrews was referring to had begun east of the Jordan as Moses administered it to them as described in the book of Deuteronomy. Even their calander was based on the first of the 11th month (crossing the Jordan) for a civil calander and the passover for the religious calander.
There were dual covenants (or several) given to Abraham. The Promise of the covenant of Mercy AND the "sign" work that was to ride along with his descendants until the covenant of mercy was fulfilled. But with the fulfilling of the Covenant of Mercy also ended the 'sign' work and also the need for geneological recordkeeping. The descendants of Israel did not circumcise in Egypt (perhaps some did) but restarted before they crossed Jordan.
The promise of individual blessing through justification by faith to all nations that was given to Abraham IS part of everlasting Life and the everlasting covenant of Mercy (Paul calls 'the promise' and the Covenant)
Abraham, Isaac, Jacob/Israel were not part of the mosaic "OLD" covenant any more than Noah was part of the 'old' covenant although they all carried the awareness and Promise that God was going to work through them and their lineage to accomplish the promise of Mercy. Their lives were ensamples of soteriology as Paul describes in Rom 9 and as is mentioned for 'law' in Ps 105.
I know that your usually very firm and well thought through with your statements and posts. But there are several points made which identify the 'old covenant' as the conditional/national covenant delivered by Moses as he spoke to them east of the Jordan just as mentioned in Duet 5:1-3. You even mentioned the 40 yr transition period during the first century which coorelates with the 40 yrs between the time of leaving Egypt (passover) and crossing the Jordan River of the formation of the 'old' covenant. It is not adequate to condense and simplify as much as possible for the sake of clarity in view of these several points: but I'd agree to put this aside for some time.
I think this is an issue that is very important in light of zionism also. The land/nation promise was prophesied to Abraham and his descendants as an absolute unconditional future occurance, but the conditional nature of keeping that land to those descendants was clarified and expressed by Moses. I think also that this was part of the association and historical context of what had occured in Babylon, and that being human nature wanting to go ahead of and usurp God using works, counterfeit religion and self-will. Israel is compared to babylon several times and in Deut 32 it describes them as sacrificing to devils.
But AFTER the desolation, I think that the christians did come back and inheirit the land, so that part of the Abrahamic promise also found favor with those of all nations.
Again, we may have to agree to disagree. You say that "the beginnings" of the 'old covenant' began with Abraham, and the prophecies and promises about it did indeed begin there, but the old covenant itself was initiated in Egypt and was later contracted by those crossing the Jordan 40 yrs later. This anti-typed and confirmed the pattern and times and seasons for the enactment and authentication of the covenant of Mercy during the 40 yrs between passover of 30 AD and the favor and protection given to the saints of all nations (Dan 7) of 70 AD.